Electronic module including a cooling substrate with fluid dissociation electrodes and related methods

ABSTRACT

An electronic module includes a cooling substrate, an electronic device mounted thereon, and a plurality of cooling fluid dissociation electrodes carried by the cooling substrate for dissociating cooling fluid to control a pressure thereof. More particularly, the cooling substrate may have an evaporator chamber adjacent the electronic device, at least one condenser chamber adjacent the heat sink, and at least one cooling fluid passageway connecting the evaporator chamber in fluid communication with the at least one condenser chamber.

FIELD OF THE INVENTION

The present invention relates to the field of electronic modules, and,more particularly, to electronic modules including a substrate forcooling one or more electronic devices and associated methods.

BACKGROUND OF THE INVENTION

Electronic devices are widely used in many types of electronicequipment. One electronic device is the integrated circuit which mayinclude a silicon or gallium arsenide substrate and a number of activedevices, such as transistors, etc. formed in an upper surface of thesubstrate. It is also typically required to support one or more suchintegrated circuits in a package that provides protection and permitsexternal electrical connection.

As the density of active devices on typical integrated circuits hasincreased, dissipation of the heat generated has become increasinglymore important. In particular, a relatively large amount of heat may begenerated in multi-chip modules (MCMs), microwave transmitters, andphotonic devices, for example.

One device which has been used in a variety of applications, includingelectronic circuit modules, to provide high thermal transport over longdistances is the so-called “heat pipe.” A heat pipe is a sealed systemthat includes an evaporator, a condenser, an adiabatic region connectingthe evaporator and condenser for liquid and vapor transport, and acapillary or wick for circulating cooling fluid therein. Heat pipesenjoy an advantage over other forms of heat regulating devices in thatthey can transfer heat without the need for a mechanical pump,compressor or electronic controls, which may provide space savings incertain instances.

An example of an MCM which uses a heat pipe is disclosed in U.S. Pat.No. 5,216,580 to Davidson et al. entitled “Optimized Integral Heat Pipeand Electronic Module Arrangement.” This MCM includes electronic circuitcomponents mounted on one side thereof and a thermal wick mounted onanother side. A heat pipe evaporator and condenser assembly is attachedto the MCM and wick assembly. Furthermore, a suitable working fluid isintroduced into the heat pipe assembly which is then hermeticallysealed.

Of course, cooling devices generally need to be on the same size scaleas the electronic devices they are intended to cool. Yet, the benefitsassociated with heat pipes are subject to scaling limitations. That is,ever increasing packaging densities, which put high power devices inclose proximity with conventional circuitry, may require that largeramounts of heat be transferred more quickly than is possible usingconventional heat pipe assemblies not having a pump.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of theinvention to provide an electronic module and related methods whichprovides adequate cooling of one or more electronic devices and hasrelatively small dimensions.

This and other objects, features, and advantages in accordance with thepresent invention are provided by an electronic module including acooling substrate, an electronic device mounted thereon, and a pluralityof cooling fluid dissociation electrodes carried by the coolingsubstrate for dissociating cooling fluid to control a pressure thereof.The cooling substrate may have an evaporator chamber adjacent theelectronic device, at least one condenser chamber adjacent the heatsink, and at least one cooling fluid passageway connecting theevaporator chamber in fluid communication with the at least onecondenser chamber.

More particularly, the electronic module may drive the plurality ofcooling fluid dissociation electrodes, for example, by sensing atemperature thereof and driving the plurality of cooling fluiddissociation electrodes responsive to the sensed temperature. Theplurality of cooling fluid dissociation electrodes may also allowcooling fluid dissociation during manufacture of the electronic module.Each of the cooling fluid dissociation electrodes may include metal, andthe metal is preferably resistant to corrosion from the cooling fluid.For example, the metal may include at least one of gold and nickel.

Furthermore, the electronic module may also include a heat sink adjacentthe cooling substrate. The plurality of cooling fluid electrodes mayinclude an evaporator thermal transfer body connected in thermalcommunication between the evaporator chamber and the electronic deviceand at least one condenser thermal transfer body connected in thermalcommunication between the at least one condenser chamber and the heatsink. The evaporator thermal transfer body each and the at least onecondenser thermal transfer body may have a higher thermal conductivitythan adjacent cooling substrate portions. Further, the evaporatorthermal transfer body and the at least one condenser thermal transferbody may have thermal conductivities greater than about 100 Watts permeter-degree Celsius.

Moreover, the evaporator thermal transfer body, the at least onecondenser thermal transfer body, and the at least one cooling fluidpassageway may cause fluid flow during operation of the electronicmodule without a pump. The evaporator thermal transfer body may includea wicking portion exposed within the evaporator chamber for facilitatingcooling fluid flow by capillary action. Also, the at least one condenserthermal transfer body may include at least one wicking portion exposedwithin the at least one condenser chamber for facilitating cooling fluidflow by capillary action.

Additionally, the cooling substrate may further include projectionsextending inwardly into the at least one cooling fluid passageway forfacilitating cooling fluid flow by capillary action. The coolingsubstrate may also include projections extending inwardly into theevaporator chamber and the at least one condenser chamber forfacilitating cooling fluid flow by capillary action.

A method aspect of the invention is for controlling cooling fluidpressure in an electronic module including a cooling substrate and anelectronic device carried by the cooling substrate. The coolingsubstrate includes an evaporator chamber, at least one condenserchamber, and at least one cooling fluid passageway connecting theevaporator chamber in fluid communication with the at least onecondenser chamber. Further, the electronic device is carried by thecooling substrate adjacent the at least one condenser chamber. Themethod includes driving a plurality of cooling fluid dissociationelectrodes carried by the cooling substrate for dissociating coolingfluid to control a pressure thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electronic module according to thepresent invention.

FIG. 2 is a cross-sectional view taken along line 2—2 of FIG. 1.

FIG. 3 is an exploded perspective view of a cooling substrate inaccordance with the invention.

FIG. 4 is a top view of the evaporator thermal transfer body of theelectronic module of FIG. 2.

FIG. 5 is a side view of the evaporator thermal transfer body of theelectronic module of FIG. 2.

FIG. 6 is a perspective view of the condenser thermal transfer body ofthe electronic module of FIG. 2.

FIG. 7 is a top view of the condenser thermal transfer body of theelectronic module of FIG. 2.

FIG. 6 is a side view of the condenser thermal transfer body of theelectronic module of FIG. 2.

FIG. 9 is a graph of modeled heat transfer capacity versus groove orcapillary wick width for the electronic module of FIG. 1.

FIG. 10 is a graph of modeled device temperature versus a number ofthermal vias used in the electronic module of FIG. 1.

FIG. 11 is a graph of junction temperature versus power dissipated forthe electronic module of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout. The dimensions of layers andregions may be exaggerated in the figures for greater clarity.

Referring initially to FIGS. 1-8, the electronic module 20 in accordancewith the invention is now initially described. The electronic module 20includes a package 21 surrounding an electronic device 22. The package21 includes a base or cooling substrate 21 a and a lid 21 b connectedthereto. The lid 21 b defines a cavity 33 over the electronic circuit 22for protection of the electronic circuit and its interfaces (not shown).The lid 21 b may be attached by brazing using a seal ring, for example,as will be appreciated by those of skill in the art, though otherconfigurations are also possible. The package 21 may include lowtemperature co-fired ceramic (LTCC) material, for example. This materialoffers advantages in terms of ruggedness, and an ability to formrecesses and small stable passageways therein, as well as to provideelectrical paths therethrough. Of course, other similar materials may beused as well.

In other embodiments, two or more electronic devices 22 may be carriedby the package 21, as will be appreciated by those skilled in the art.The electronic device 22 may include semiconductor devices or integratedcircuits, heat coils, resistors, etc., for example. Of course, otherelectronic devices may also be included in the electronic module 20. Thepackage 21, as best seen in FIG. 1, may carry electrical connectors 34on at least one of its surfaces. For example, the electrical connectors34 may be pins in a pin grid array, as illustratively shown. In otherembodiments, edge connectors may be provided to connect to a ribbon typecable, for example, as will be appreciated by those skilled in the art.

A heat sink 23 is adjacent the cooling substrate 21 a and may includefins 24, for example. Of course, other heat sinks known to those ofskill in the art may also be used. For example, the heat sink maybe arack or metal chassis in which the electronic module is mounted.Further, the heat sink and one or more condenser thermal transfer bodies(described below) may be formed as a single homogenous body. The coolingsubstrate 21 a also has an evaporator chamber 25 adjacent the electronicdevice 22, at least one condenser chamber 26 adjacent the heat sink 23,and at least one cooling fluid passageway 27 connecting the evaporatorchamber in fluid communication with the at least one condenser chamber.The total area of the condenser chambers 26 may be made grater than thatof the evaporator chamber 25 to reduce heat flux entering the fins (orheat sink) 24. This may be particularly desirable to do for electronicdevices 22 that get so hot that even direct attachment to a heat sinkwould be insufficient to properly cool the device.

In the embodiment illustrated in FIG. 3, the cooling substrate 21 aincludes four condenser chambers 26 and four cooling fluid passageways27 extending radially outward from the evaporator chamber 25 in aconfiguration resembling an “X”. Of course, those of skill in the artwill appreciate that any number of cooling fluid passageways andcondenser chambers may be used without departing from the scope of thepresent invention.

The electronic module 20 further includes an evaporator thermal transferbody 28 connected in thermal communication between the evaporatorchamber 25 and the electronic device 22. Furthermore, a condenserthermal transfer body 36 is connected in thermal communication betweeneach condenser chamber 26 and the heat sink 23. Of course, it willappreciated that more than one heat sink 23 may be used in accordancewith the invention.

The evaporator thermal transfer body 28 and the condenser thermaltransfer bodies 36 each preferably have thermal conductivities greaterthan about 100 Watts per-meter degree Celsius. The thermal transferbodies 28, 36 may also have a higher thermal conductivity than adjacentportions of the cooling substrate 21 a. Those skilled in the art willappreciate that the thermal transfer bodies 28, 36 allow a low operatingtemperature of the electronic device 22 to be maintained, as illustratedin FIG. 10.

By way of example, the evaporator thermal transfer body 28 and thecondenser thermal transfer bodies 36 may include at least one of acopper-graphite composite material, AlSiC, and metal. Of course, othersuitable materials known to those of skill in the art may also be used,and it is preferred that the material used be resistant to corrosionfrom the cooling fluid (e.g., at least one of nickel and gold). It isanticipated that both thermal transfer bodies 28, 36 will be used inaccordance with the present invention to maximize cooling fluid flow.Yet, it should be understood that both thermal transfer bodies may notbe necessary in every application and that one or the other may be used.

As a result, the evaporator thermal transfer body 28, the condenserthermal transfer bodies 36, and the cooling fluid passageways 27 causecooling fluid flow during operation of the electronic module without apump. Moreover, the various materials noted above that are used for thecooling substrate 21 a and the thermal transfer bodies 28, 36 are allreasonably matched in temperature coefficient of expansion (CTE) (amaterial property) with each other and semiconductor materials. Thismatching allows for direct mounting of silicon and other electronicdevices 22 to the cooling substrate 21 a. For example, the coolingsubstrate 21 a may be an LTCC and the thermal transfer bodies 28, 36 maybe a copper-graphite composite material.

To enhance the cooling fluid flow, the evaporator thermal transfer body28 includes a wicking portion exposed within the evaporator chamber 25for facilitating cooling fluid flow by capillary action. The wickingportion includes a plurality of projections 30 extending outwardly froma base plate 29. The plurality of projections 30 may be arranged in agenerally rectangular pattern, as shown in FIGS. 4 and 5, although otherconfigurations may also be used. The base plate 29 facilitates sealingwith adjacent cooling substrate 21 a portions, as will be describedfurther below.

Likewise, each condenser thermal transfer body 36 also includes at leastone wicking portion exposed within its respective condenser chambers 26for facilitating cooling fluid flow by capillary action. The wickingportion includes a base 45 and a plurality of projections 38 extendingoutwardly therefrom. The projections 38 may be arranged in two generallyrectangular groups oriented at a substantially right angle, asillustrated in FIGS. 6 and 7. Also, the condenser thermal transfer body36 may further include a base plate 37 for facilitating sealing withadjacent cooling substrate 21 a portions, as will be described furtherbelow. Each of the projections 38 may include a reduced width tipportion 39 to alleviate capillary flooding by increasing the gapdistance therebetween. This facilitates the removal of a thermallyimpeding fluid layer and thus promotes condensation. The condenserthermal transfer bodies 36 may also include a reservoir portion 40adjacent the wicking portion defining a cooling fluid reservoir.

Further, to enable cooling fluid flow return to the evaporator chamber25, the cooling substrate 21 a may also include projections 41 extendinginwardly into the cooling fluid passageways, the evaporator chamber, andthe condenser chambers 26 defining respective wicking surfaces forfacilitating cooling fluid flow by capillary action. That is, a highflow rate capillary is formed with a high surface energy that providesinternal full surface coverage and allows for rapid fluid transport.This alleviates capillary depletion which otherwise may occur onnon-wicking surfaces, which may be particularly important in small heatpipe assemblies.

The projections 41 may be created, for example, by forminginterconnecting orthogonal grooves on adjacent cooling substrate 21 alayers on opposing parallel surfaces of the cooling fluid passageways 27and chambers 25, 26. Processes used to create these structures are basedon standard multilayer ceramic fabrication techniques known to those ofskill in the art. Of course, those skilled in the art will appreciatethat other wicking structures may also be used in accordance with thepresent invention.

In addition, a plurality of fluid dissociation electrodes may be carriedby the cooling substrate 21 a to control a pressure of the coolingfluid. In one embodiment, any two of the thermal transfer bodies 28, 36may be used as the fluid dissociation electrodes and may be driven bydiffering (e.g., positive and negative) DC potentials (see FIG. 2), forexample, to cause dissociation of cooling fluid. In an alternativeembodiment, fluid dissociation electrodes 42 may be mounted within thecooling substrate 21 a. By causing cooling fluid to dissociate into itsconstituent gases, the fluid dissociation electrodes allow the pressure,and thus the flow rate, of the cooling fluid to be controlled. That is,the partial pressure of the dissociated gases are increased, whichthereby decreases the partial pressure of the fluid vapor and increasesthe operating temperature. When the fluid dissociation electrodes areused, the cooling fluid is preferably one that may be dissociated when acurrent is passed therethrough, for example, water.

In one embodiment, the fluid dissociation electrodes allow dissociationof cooling fluid during the manufacturing process. In anotherembodiment, the electronic device 22 may drive the fluid dissociationelectrodes 42 (or the thermal transfer bodies), for example, by sensinga temperature of the electronic device and driving the electrodesresponsive to the sensed temperature, as illustratively shown with adashed connection line 46 in FIG. 2. Each of the fluid dissociationelectrodes 42 may be metal which, again, is preferably resistant tocorrosion from the cooling fluid. The electrodes 42 may therefore alsoinclude at least one of gold and nickel. Of course, it will beappreciated by those skilled in the art that the fluid dissociationelectrodes 42 may be used in a variety of structures other than thecooling substrate disclosed herein.

The electronic module 20 may be fabricated as a multi-layer ceramicstructure as follows. Unfired (green) ceramic in tape form is processedto cut the required cavities and wick structures. Internal thick filmconductors are then printed as required to route the multi-levelcircuitry. Once the individual layers are complete, they are stacked andlaminated to form a green body which is then fired. Singulation ofindividual structures may then be performed, if necessary, by diamondsawing or other suitable methods. Lapping may be required to removesurface deformations produced during lamination and firing, as will beappreciated by those of skill in the art.

Post-fired thick film conductors are then printed and fired to providethe base plates 29, 37 to provide brazeable, sealing surfaces for theevaporator and condenser thermal transfer bodies 28, 36, respectively.The base material for the thermal transfer bodies 28, 36 (e.g., acopper-graphite composite material) may then be nickel and gold platedfor brazing to the LTCC cooling substrate 21 a. The brazing process ispreferably fluxless to avoid contamination of the wicking surfaces andprovides hermetic sealing of the cooling substrate 21 a.

Fine leak verification may be used to ensure package integrity, as willbe appreciated by those skilled in the art. It may also be necessary toattach the electronic device 22 prior to filling the cooling substrate21 a with cooling fluid to allow for solder die to attach to the thermaltransfer bodies 28, 36. Otherwise, a filled, operational heat pipe mayinhibit reflow or catastrophically fail if internal vapor pressuresbecome to high. A fill tube, such as a copper tube, may be included inthe cooling substrate 21 a for evacuation and filling. The filling maybe accomplished by injection.

Those of skill in the art will appreciate that numerous advantages areprovided according to the present invention. For example, the enhancedcapillary flow achieved according to the present invention may allow forminiature cooling fluid channel dimensions believed to be previouslyunavailable in the prior art. Also, the fluid reservoir 40 andevaporator thermal transfer body 28 incorporating the wicking portionreduces the effect of pool boiling and extends the power density upperlimit.

Moreover, the condenser thermal transfer body 36 promotes clearing ofthe condensate (liquid) from the condensing surface to providesubstantially continuous unimpeded condensation. Also, designs includinga central or multiple evaporator chambers 25 and one or more condenserchambers 26 interconnected by one or more cooling fluid passageways 27for the routing of cooling fluid are also provided by the presentinvention. This enables dense packaging of large heat flux devices andstabilizes the temperature of multiple components at identicaltemperatures. Additionally, as noted above, the dissociation electrodes42 allow for gas generation and, consequently, regulation of pressureand operating temperature.

The preceding advantages may be more fully appreciated with reference tothe test results provided in the following example of an electronicmodule fabricated according to the present invention.

EXAMPLE

The design of the electronic module 20 according to the presentinvention accommodates heat spreading away from the electronic device 22as well as temperature stable operation, as will be appreciated by thoseof skill in the art. Spreading is accomplished by increasing thecondenser area with respect to that of the evaporator and by dividingand separating the condenser from the evaporator. For example, a testelectronic module according to the present invention was constructed tohave a total condenser area of about twice that of the evaporator area,although other dimensions may also be used according to the invention.

Each segment of the heat pipe structure defined by the cooling substrate21 a has unique considerations, several of which relate to scalinglimitations. The most notable limitation is that such “miniature” heatpipes are more sensitive to vapor-liquid interaction through theadiabatic region than their larger counterparts. Vapor flow can bedramatically restricted if certain characteristic dimensions are notmaintained. For the test device described herein, it was determined thata minimum vapor channel cross section of 1.27 mm was required, thoughsmaller dimensions may be possible according to the present invention inother designs, as will be understood by those of skill in the art.

A low partial pressure of any unintended gasses should be maintained toavoid reducing the partial pressure of the vapor formed from the desiredcooling fluid. Miniature heat pipes are very sensitive to unintendedgasses, and thus hermetic sealing is preferred. Furthermore, the wickingsurfaces 41 should provide a fine enough structure to preventcondensation droplets from forming and depleting the cooling fluidsupply. Additionally, cooling fluid should come in close contact withthe heat sources to enable temperature stable device operation, andcondensation surfaces should remain clear of a thermally impeding fluidlayer. Moreover, capillary flow should be unimpeded with sufficientcapacity, and a condenser-to-evaporator ratio should be sufficientlyhigher than one. The choice of cooling fluid is also important as itshould be free of contaminants, especially dissolved gasses. Water waschosen for the cooling fluid in the test device because of its highlatent heat of evaporation, well-understood properties, and ease ofdegassing by boiling. Of course, other cooling fluids may also be used.

Other design parameters may also need to be taken into account. Forexample, some of the key geometric parameters include thecross-sectional structure of the cooling fluid passageways 27 and lengthof the wicking surfaces 41. Others parameters include the evaporator andcondenser chamber 25, 26 sizes and structures, which may be driven byempirical results and fabrication issues, as will be understood by thoseof skill in the art. These parameters drive the size of the devicerequired to provide a given heat dissipation capacity for a given typeand number of electronic devices 22. Further discussion of such designparameters with respect to the present invention may be found in a paperentitled “Miniature Embedded Heat Pipes in Low Temperature Co-FiredCeramic for Electronic Devices Requiring Temperature Stability,” by theapplicants of the present invention which was presented at the Societyof Automotive Engineers Aerospace Power Systems Meeting on Nov. 1, 2000,in San Diego, Calif., which is hereby incorporated herein in itsentirety by reference.

Generally speaking, the capillary-action pump is the limiting factor inheat transport capability. One of the key parameters for capillarypumping is the width of the grooves used to define the projections 41and resulting wicking surfaces. This parameter is important because itdrives the capillary limit to supply cooling fluid to the evaporatorchamber 25. The groove width should be carefully chosen to account forsag and layering requirements, as will be appreciated by those of skillin the art. The groove width chosen for the test device (about 4 mils)was selected based upon materials and fabrication issues, includingconsiderations of total substrate thickness. Guidance from the modelingdiscussed further in the above referenced paper was also used. A curveshowing the optimization of heat transport due to the capillary andvapor friction resulting from variation of the groove width may be seenin FIG. 10.

Another key parameter of importance is the heat transport length fromthe heat source (i.e., electronic device 22) to the evaporator chamber25. This is important for the effectivity of the cooling substrate 21 ato maintain a moderately low temperature on the electronic device 22requiring the heat dissipation. The electronic device 22 shouldinterface with the evaporator chamber 25 with as little thermalresistivity as possible. As such, it is preferable that any materialused underneath the electronic device 22 be vacuum sealable to the LTCCand matched in CTE, as discussed above.

A simplified model may be used to illustrate thermal resistivity of theLTCC cooling substrate 21 a with thermal vias for conduction to theevaporator chamber 25. The model includes a thermal path through asubstrate of the electronic device 22 and the cooling substrate 21 awith thermal vias (each of which has its own thermal resistivity) andterminating in an idealized constant-temperature evaporator chamber.

Based upon this model, while the evaporator may function at around 45°C., the source device temperature may be significantly higher, dependingon the materials and geometry, as may be seen in the graph of FIG. 10.The independent variable represents the number of thermal vias in thepath directly beneath the electronic device 22. The graph alsoillustrates that the electronic device 22 temperature cannot bemaintained at a constant value versus heat load if there is significantthermal resistivity in the thermal path. Thermal resistivity isparticularly important if the goal is to eliminate thermal electriccoolers from the system. With an ambient environment, the goal istypically to keep electronic devices operating as close to ambient aspossible.

With the above design considerations in mind, the test device wasfabricated to include four condenser chambers 26 and four cooling fluidpassageways 27 connecting respective condenser chambers to theevaporator chamber 25. Each of the cooling fluid passageways was made9.5 mm in length, though longer or shorter lengths are possibleaccording to the present invention. Again, a small hole was formed onthe top side of the electronic module 20 just over one of the condenserchambers 26 and a copper fill tube was also brazed therein to allowevacuation and filling of the cooling fluid.

The thermal transfer bodies 28, 36 were mounted to allow direct couplingto the respective wicking surfaces 41. The thermal transfer bodies 28,36 were hermetically brazed to the LTCC cooling substrate 21 a using80/20 gold/tin. An Ultra _FETTm Power metal oxide semiconductor fieldeffect transistor (MOSFET) bare die (0.28 cm) made by the assignee ofthe present invention was used as the electronic device 22. This MOSFETwas solder mounted adjacent the evaporator chamber 25 and wire bonded toa thick film surface metallization forming the base plate 29 to provideelectrical interconnect. Wires soldered to the same metallizationprovided interconnection to an electrical test fixture.

As noted above, the cooling substrate 21 a may be LTCC. LTCC is acommercially available low temperature firing glass-ceramic (850° C.)system originally developed for multi-layered circuit fabrication whichcan accommodate high conductivity metal circuits (such as gold, silver,and copper) and hermetic packaging. Typical properties of the tape notedabove include a thermal conductivity between 2 and 3 W/mK and a CTE of 7ppm/°C. High thermal conductivity thermal transfer bodies 28, 36 with aCTE matched to that of the LTCC were used to seal the heat pipes at theevaporator and condenser chambers 25, 26, and 80/20 Gold/Tin solder wasused to provide hermeticity.

In order to test the device, several pieces of test equipment were usedincluding a DC power supply, a custom power control circuit, athermoelectric cooler, and two 0.003″ wire type K thermocouples withtemperature meters. The custom power control circuit allowed forindependent control of the MOSFET power utilizing feedback circuitry,and the thermoelectric cooler was used to maintain a stable condensertemperature. The thermoelectric cooler was maintained at 20° C.throughout the test to provide a stable condenser chamber 26 temperaturewhich aided measurement taking and calculations. One of the twothermocouples was used to measure the condenser temperature, and theother was placed in contact with the MOSFET surface with the aid ofthermal grease which allowed continuous junction temperature monitoring.All tests were conducted horizontally in still air.

Test units fabricated as described above were tested over a range of 10Watts of heat dissipation. The results may be seen in the graph shown inFIG. 5. The graph illustrates a junction temperature runaway thatresults when the substrate is passive (i.e., unfilled) (line 46) versusan active operating substrate (line 47). A hystersis reference line 48is also provided. The thermal heat pipe cycle activated at about 3 Wattsand stabilized at about 6 Watts. This range of parameters may be idealfor many electronic devices or other systems requiring reduced powerdissipation and reduced circuit complexity. More specifically, thepresent invention is therefore suitable for use with laser diode arrays,computer central processing unit (CPU) chips, radio frequency (RF) powermodules, high density multi-chip modules, optical modules, and phasedarray antennas, for example.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed, and that othermodifications and embodiments are intended to be included within thescope of the appended claims.

That which is claimed is:
 1. An electronic module comprising: a coolingsubstrate and an electronic device mounted thereon; said coolingsubstrate having an evaporator chamber adjacent said electronic device,at least one condenser chamber, and at least one cooling fluidpassageway connecting said evaporator chamber in fluid communicationwith said at least one condenser chamber; and a plurality of coolingfluid dissociation electrodes carried by said cooling substrate fordissociating cooling fluid to control a pressure thereof.
 2. Theelectronic module of claim 1 wherein said plurality of cooling fluiddissociation electrodes allow cooling fluid dissociation duringmanufacture of the electronic module.
 3. The electronic module of claim1 wherein said electronic device drives said plurality of cooling fluiddissociation electrodes.
 4. The electronic module of claim 3 whereinsaid electronic device senses a temperature thereof and drives saidplurality of cooling fluid dissociation electrodes responsive to thesensed temperature.
 5. The electronic module of claim 1 wherein each ofsaid cooling fluid dissociation electrodes comprises metal.
 6. Theelectronic module of claim 5 wherein the metal is resistant to corrosionfrom the cooling fluid.
 7. The electronic module of claim 6 wherein themetal comprises at least one of gold and nickel.
 8. The electronicmodule of claim 1 further comprising a heat sink adjacent said coolingsubstrate; and wherein said plurality of cooling fluid dissociationelectrodes comprise an evaporator thermal transfer body connected inthermal communication between said evaporator chamber and saidelectronic device and at least one condenser thermal transferbodyconnected in thermal communication between said at least onecondenser chamber and said heat sink.
 9. The electronic module of claim8 wherein said evaporator thermal transfer body and said at least onecondenser thermal transfer body each have a higher thermal conductivitythan adjacent cooling substrate portions.
 10. The electronic module ofclaim 8 wherein said evaporator thermal transfer body and said at leastone condenser thermal transfer body each have a thermal conductivitygreater than about 100 Watts per meter-degree Celsius.
 11. Theelectronic module of claim 8 wherein said evaporator thermal transferbody, said at least one condenser thermal transfer body, and said atleast one cooling fluid passageway cause fluid flow during operation ofthe electronic module without a pump.
 12. The electronic module of claim8 wherein said evaporator thermal transfer body comprises a wickingportion exposed within said evaporator chamber for facilitating coolingfluid flow by capillary action.
 13. The electronic module of claim 8wherein said at least one condenser thermal transfer body comprises atleast one wicking portion exposed within said at least one condenserchamber for facilitating cooling fluid flow by capillary action.
 14. Theelectronic module of claim 8 wherein said cooling substrate furthercomprises projections extending inwardly into said at least one coolingfluid passageway for facilitating cooling fluid flow by capillaryaction.
 15. The electronic module of claim 8 wherein said coolingsubstrate further comprises projections extending inwardly into saidevaporator chamber and said at least one condenser chamber forfacilitating cooling fluid flow by capillary action.
 16. An electronicmodule comprising: a cooling substrate and an electronic device mountedthereon; a heat sink adjacent said cooling substrate; said coolingsubstrate having an evaporator chamber adjacent said electronic device,at least one condenser chamber adjacent said heat sink, and at least onecooling fluid passageway connecting said evaporator chamber in fluidcommunication with said at least one condenser chamber; and a pluralityof cooling fluid dissociation electrodes carried by said coolingsubstrate for dissociating cooling fluid to control a pressure thereof,at least one of said plurality of fluid dissociation electrodescomprising an evaporator thermal transfer body connected in thermalcommunication between said evaporator chamber and said electronicdevice.
 17. The electronic module of claim 16 wherein said plurality ofcooling fluid dissociation electrodes allow cooling fluid dissociationduring manufacture of the electronic module.
 18. The electronic moduleof claim 16 wherein said electronic device drives said plurality ofcooling fluid dissociation electrodes.
 19. The electronic module ofclaim 18 wherein said electronic device senses a temperature thereof anddrives said plurality of cooling fluid dissociation electrodesresponsive to the sensed temperature.
 20. The electronic module of claim16 wherein each of said cooling fluid dissociation electrodes comprisesmetal.
 21. The electronic module of claim 20 wherein the metal isresistant to corrosion from the cooling fluid.
 22. The electronic moduleof claim 21 wherein the metal comprises at least one of gold and nickel.23. The electronic module of claim 16 wherein said evaporator thermaltransfer body comprises a wicking portion exposed within said evaporatorchamber for facilitating cooling fluid flow by capillary action.
 24. Theelectronic module of claim 16 wherein said evaporator thermal transferbody has a higher thermal conductivity than adjacent cooling substrateportions.
 25. The electronic module of claim 16 wherein said evaporatorthermal transfer body has a thermal conductivity greater than about 100Watts per meter-degree Celsius.
 26. An electronic module comprising: acooling substrate and an electronic device mounted thereon; a heat sinkadjacent said cooling substrate; said cooling substrate having anevaporator chamber adjacent said electronic device, at least onecondenser chamber adjacent said heat sink, and at least one coolingfluid passageway connecting said evaporator chamber in fluidcommunication with said at least one condenser chamber; and a pluralityof cooling fluid dissociation electrodes carried by said coolingsubstrate for dissociating cooling fluid to control a pressure thereof,at least one of said plurality of cooling fluid dissociation electrodescomprising a condenser thermal transfer body connected in thermalcommunication between said at least one condenser chamber and said heatsink.
 27. The electronic module of claim 26 wherein said plurality ofcooling fluid dissociation electrodes allow cooling fluid dissociationduring manufacture of the electronic module.
 28. The electronic moduleof claim 26 wherein said electronic device drives said plurality ofcooling fluid dissociation electrodes.
 29. The electronic module ofclaim 28 wherein said electronic device senses a temperature thereof anddrives said plurality of cooling fluid dissociation electrodesresponsive to the sensed temperature.
 30. The electronic module of claim26 wherein each of said cooling fluid dissociation electrodes comprisesmetal.
 31. The electronic module of claim 30 wherein the metal isresistant to corrosion from the cooling fluid.
 32. The electronic moduleof claim 31 wherein the metal comprises at least one of gold and nickel.33. The electronic module of claim 26 wherein said condenser thermaltransfer body comprises at least one wicking portion exposed within saidat least one condenser chamber for facilitating cooling fluid flow bycapillary action.
 34. The electronic module of claim 26 wherein saidcondenser thermal transfer body has a higher thermal conductivity thanadjacent cooling substrate portions.
 35. The electronic module of claim26 wherein said condenser thermal transfer body has a thermalconductivity greater than about 100 Watts per meter-degree Celsius. 36.A method for controlling cooling fluid pressure in an electronic modulecomprising a cooling substrate having an evaporator chamber, at leastone condenser chamber, and at least one cooling fluid passagewayconnecting the evaporator chamber in fluid communication with the atleast one condenser chamber and an electronic device carried by thecooling substrate adjacent the at lest one condensor chamber, the methodcomprising: driving a plurality of cooling fluid dissociation electrodescarried by the cooling substrate for dissociating cooling fluid tocontrol a pressure thereof.
 37. The method of claim 36 wherein drivingcomprises driving the plurality of cooling fluid dissociation electrodesusing the electronic device.
 38. The method of claim 37 wherein drivingcomprises sensing a temperature of the electronic device and driving theplurality of cooling fluid dissociation electrodes responsive to thesensed temperature.
 39. The method of claim 36 wherein each of thecooling fluid dissociation electrodes comprises metal.
 40. The method ofclaim 39 wherein the metal is resistant to corrosion from the coolingfluid.
 41. The method of claim 36 wherein the metal comprises at leastone of gold and nickel.
 42. The method of claim 36 further comprisingconnecting a heat sink to the cooling substrate adjacent the at leastone condenser chamber.
 43. The method of claim 42 wherein at least oneof the plurality of cooling fluid dissociation electrodes comprises acondenser thermal transfer body in thermal communication between the atleast one condenser chamber and the heat sink.
 44. The method of claim42 wherein at least one of the plurality of cooling fluid dissociationelectrodes comprises an evaporator thermal transfer body in thermalcommunication between the evaporator chamber and the electronic device.